1. Field of the Invention
The present invention relates to an optical deflector wherein a beam of light made incident thereupon is deflected and made emergent therefrom, and a display unit for displaying an image by using the optical deflector.
2. Description of Background Art
Conventionally, optical deflectors having various arrangements are known.
FIGS. 14 and 15 illustrate an operating principle of an optical deflector described, for instance, on page 86 and onwards of the "Optical Communication Circuit and System" (1st edition published by OHMSHA, LTD. on Feb. 25, 1987).
This optical deflector is an optical switch operated by changing the angle of a rhomboid prism.
Specifically, the angle of a rhomboid prism 1 provided in this optical deflector is changed between a first position shown in FIG. 14 and a second position shown in FIG. 15 by an unillustrated driving device using an electromagnet or the like.
In the first position, light beams 2-1 and 2-2 are made incident perpendicularly to a side 1a of the rhomboid prism 1. In this case, the light beams 2-1 and 2-2 advance straight through the interior of the rhomboid prism 1 and are made emergent from a side 1b opposing the side 1a.
In the second position, the light beams 2-1 and 2-2 are made emergent upon the rhomboid prism 1 in such a manner as to straddle an apex 1c of the rhomboid prism 1. In this case, the light beams 2-1 and 2-2, when made incident, are refracted, intersect each other inside the rhomboid prism 1, and are made emergent from the vicinity of an apex 1d opposing the apex 1c in a vertically inverted positional relationship, as viewed in the drawing, compared with the positional relationship of incidence.
It should be noted that the rhomboid prism 1 has a greater refractive index than the atmosphere.
Such an optical deflector is used in, for instance, the field of optical communication as a 2 .times. 2 optical switch for supplying incident light beams of two systems by changing them between two optical paths, respectively.
Meanwhile, optical deflectors for repeatedly scanning a fixed range are also known.
In the optical deflector using the aforementioned rhomboid prism 1, if the rhomboid prism 1 is rotated, the direction of emergence of the light beam is not displaced in a fixed cycle, and the range of displacement changes each time a side relating to the incidence changes. Since this type of optical deflector is not suitable for repeated scanning, optical deflectors which will be described below are used in telecine apparatuses requiring such scanning.
Optical deflectors suitable for repeated scanning are described in, for instance, "4.1 Applications of Laser Scanning Technology to Information Equipment" (The Journal of the Institute of Electronics and Communication Engineers of Japan, p. 372 , Apr. 1985 edition). This optical deflector employs a rotating polygonal mirror, a galvanometer, or a rotating hologram plate.
Optical deflectors having such arrangements are used for laser printers, POS scanners, telecine apparatuses for converting an image of a motion picture into a television signal, or other similar equipment.
However, the following drawbacks are experienced with the aforementioned types of optical deflectors used for repeated scanning.
First, the optical deflector using a rotating polygonal mirror realizes repeated scanning by making use of an angular change of a reflected light beam resulting from the rotation of the rotating polygonal mirror. Therefore, an angle double an apex angle of a rotating polygonal mirror is obtained as a deflection angle of repeated scanning. For instance, in a case where a deflection angle of 30 degrees or thereabouts is required, a 25-sided mirror is required as the rotating polygonal mirror. In order to enhance the accuracy of the deflection angle in such a rotating polygonal mirror having a large number of apexes, it is necessary to increase the diameter of the rotating polygonal mirror by a certain degree, e.g., to 4-6 cm or thereabouts, for purposes of processing. This brings about disadvantages in that the optical deflector becomes large in size and that the driving electric power for the rotating polygonal mirror becomes large.
In the case of the optical deflector using the galvanometer, it is possible to make the arrangement compact as compared with the optical deflector using a rotating polygonal mirror. However, since an electromagnetic means is used to drive the mirror, a limit to a scanning cycle is 1 kHz or thereabouts, so that high-speed scanning is difficult.
In addition, with the optical deflector using the rotating hologram plate, the optical system can be realized with a simple arrangement, and this optical deflector is advantageous in that mass-production efficiency of rotating hologram plates is good. However, this type of optical deflector has drawbacks in that the scanning path is not rectilinear but arcuate and that the chromatic aberration is large.